A coral island is a type of island formed from coraldetritus and associated organic material.[1] They occur in tropical and sub-tropical areas, typically as part of coral reefs which have grown to cover a far larger area under the sea.

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Coral reefs are some of the oldest ecosystems on the planet. Over geological time forming massive reefs made of limestone. The reef environment supports more plant and animal species than any other habitat. Coral reefs are vital for life for multiple aspects some of which include structure, ecology, and nutrient cycles which all support biodiversity in the reefs.

The reefs build massive calcareous skeletons that serve as homes for animals from fish hiding inside the crooks and crannies of the reef and barnacles attaching themselves directly to the coral’s structure. The reef’s structure also helps with plants that need the sun to photosynthesize; by lifting the plants to the ocean’s surface where the sunlight can penetrate the water. The structure of the reef also creates a calm zone in the ocean providing a great place for fish and plant species to thrive.

Over geological time the reefs reach the surface and can become a coral island, where it begins a whole new ecosystem for land based creatures.[2]

Volcano with fringing reef, to barrier reef, and finally formation into a coral atoll

Coral islands begin as a volcanic island over a hot spot. As the volcano emerges from the sea a fringing reef grows on the outskirt of the volcano. The volcano eventually moves off of the hot spot through a process known as plate tectonics. Once this occurs the volcano can no longer keep up with the erosion that is taking place due to the ocean and undergoes subsidence.

Once the island is submerged the coral must keep growing to stay in the epipelagic zone (sunlit). This causes the coral to grow into an atoll with a shallow lagoon in the middle. The lagoon can then undergo accretion and create an island completely made of carbonate materials.The process is later enhanced with the remains of plant life which grows on the island.[3]

Coral is important for biodiversity and the growth of fish populations, so maintaining coral reefs is important. Coral reefs are threatened by numerous anthropogenic impacts, some of which have already had major effects worldwide.[4] Reefs grow in shallow, warm, nutrient poor waters where they are not out competed by phytoplanktons. By adding fertilizers into the water runoff phytoplankton populations can explode and choke out coral reef systems. Adding too many sediments can cause a similar problem by blocking out the sun starving the zooxanthellae that live on coral causing it to undergo a process known as coral bleaching. The oceans acidity is also a factor. Coral is made of calcium carbonate and is dissolved by carbonic acid. With the increase in carbon dioxide from combustion reactions in the atmosphere through precipitation carbon dioxide mixes with water and forms carbonic acid raising the oceans acidity which slows coral growth. Through chemical and physical changes humans can cause significant harm to reef systems and slow the creation of coral island chains.[2]